Counter Rotating Motor for Catamarans

[Southern Salt]

I'm looking at re-powering my 2000 Twin Vee 22' center console. It's currently powered by twin 2000 90 hp Johnson two strokes, non-counter-rotating, with 20" shafts. I'm hoping to re-power with counter-rotating 115 hp Yamaha four strokes. The reason I'd like to go with the counter-rotating motors is that the boat leans to port while running, due to the torque of the non-counter-rotating motors. I can overcome this by trimming the starboard motor up, but then I mostly lose the ability to trim the port motor without again causing the boat to list while running. The problem I have is that Yamaha only makes a counter-rotating 115hp in a 25" shaft. An outboard dealer has recommended going with the 25" shaft counter-rotating motors and using manual jack plates to compensate for the added shaft length. Would there be any potential issues introduced by running the boat with the longer shafts with the jack plates or does anyone have any other ideas?? Thank you for any suggestions.

[dacust]

I can't speak to your exact situation. But in general, with a jack plate, there's a couple things to take into account.

It will change the geometry of the torque applied to your motor mount. It only takes a very few inches to drastically increase the torque. You might contact the manufacturer and ask them if a jack plate with x" set-back and y" lift and n hp should be OK. Otherwise, check with a performance speedboat shop and see what they think. If your boat is rated for much higher engines than you are using, you got a better chance it will be fine.

Also, a jack plate will add a set-back. Probably not a factor in your case, but I'll mention it anyway. The set-back is normally only a very few inches. This can put the prop at a different place in the water hump caused by the hull. This can help or hinder. For high performance boats, we always tried to get the motor hitting the water a bit (that's a technical term) behind the highest point of the hump. An efficient sailboat hull probably does not have enough of a hump to be a factor. The set-back can also change the mounting torque, but I can't imagine in your situation it would be enough to be a factor, either.

The torque factor is the biggie. Most quality boats should be able to handle the extra torque added by a jack plate. But it would be embarrassing to be offshore in nasty weather, hit the throttle hard to straighten up while climbing a huge wave, and suddenly have one of the engines unceremoniously part with your company, taking a large chunk of the transom with it.

-dan

[Jesse]

It has been a lot of years ago but I remember reversing the rotation on Johnson and Evinrude two-stroke motors by changing timing to the other side of TDC then reversing the hoses on the water pump and having the starter reversed. They worked just as well in either direction. Perhaps it could still be done?

[S/V Alchemy]

I can't speak to powerboat physics, but in World War II, twin engined fast fighters like the Mosquito had contra-rotating engines to avoid yaw when trundling down the runway. So your principles are sound. Whether it can be done is another issue.

[Southern Salt]

Thank you so much for the information. I am waiting a response from the manufacture on this issue. I am having a hard time finding information about counter rotating motor on cats. I have heard that it is done on the larger boats, but also can't find it on the web. Thanks again,

Brad

[aclmck]

The other area to keep an eye on is all the extra weight you are adding to this outfit with plates you could be up over80-100 kgs this can interfere with both the stability and performance, alternatives maybe in Honda or suzuki

[aclmck]

Lots of the larger cats have the motors far enough apart it then is not an issue,Mine were 3.6m apart so no issue

[dacust]

Counter-rotating also neutralizes prop walk. Or, even better, when maneuvering, you can favor the engine that gives you the prop walk in the direction you want. This is less noticeable on an outboard where the prop shaft is parallel with the surface. But on inboards it can make a big difference.

I would prefer counter rotating on any twin engine boat. I COULD however see forgoing them if the reverse spinning one was so much more expensive that the advantages weren't worth it.

-dan

[Southern Salt]

I am thinking the same thing, but the boat builders are hard to contact about the issue. My motors are 8 Ft apart. I think the reason they did not put counter rotating props on it the first time is 10 years ago the counter rotating was not made in my smaller motors.

Thanks again,

Brad

[Cheechako]

If your boat is a deep V design, the counter rotators may not help your problem. Hard to say. I worked for a boat manufacturer that had the same problem on a lot of boats in teh 23-26 ft range, deep V design. We almost always put on counter rotating engines. The problem prevailed, worse on some boats. Did a lot of testing, changing strake design etc. Never really solved it. For a while I thought it was a prevailing wind issue, because when we changed course, sometimes it went away.. arghh!

[Searles]

Most counter rotation is done in the g boxes ,both crank shafts rotate the same way,verry slight change in engine revs ,most 2stroke petrol and Diesel engines can be reversed in crankshaft direction gear ratio in 4stroke outboards maybe altered in the g box.hope this helps ⚓️

[Searles]

Scrap that ,didn’t read date ,sorry

[Dockhead]

Originally Posted by S/V Alchemy

I can't speak to powerboat physics, but in World War II, twin engined fast fighters like the Mosquito had contra-rotating engines to avoid yaw when trundling down the runway. So your principles are sound. Whether it can be done is another issue.

Not just for the runway. With counter-rotating props, you can turn in either direction with equal ROT because you don't have torque favoring one direction -- important for a fighter. Less trim needed to counteract torque, especially when climbing when P factor comes into play.